Uranium groundwater monitoring and seismic analysis into the Gran Sasso hydrogeological basin

Abstract

Nowadays, the e cacy of many observables proposed as possible seismic pre- cursory signals is, at most, unproven, mainly due to the lack of appropriate, systematic and longterm observations. In fact, strong earthquakes rarely occurs and every presumed precursor can be often a ected by its own variability, that it is not linked to seismic features. This PhD Thesis is based on the identi cation and characterization of possible anomalies in uranium (U) groundwater content and on the correlation between this observable and the seismicity of the sampling area. Such an analysis is one of the rst studies in this research eld available so far, mainly due to the uniqueness of the achievable observations. In fact, uranium groundwater content has been monitored weekly within the framework of the INFN's scienti c program ERMES (Environmental Radioactivity Monitoring for Earth Sciences) since June, 2008. The area under investigation was a ected by a seismic swarm from October, 2008 to December, 2009, with the main shock occurring at 01:33 UT on April 6th, 2009 (Ml=5.9). Gran Sasso National Laboratories (LNGS-INFN) are located inside the lar- gest aquifer in central Italy and within the limestone formation of the upturned syncline, near the main overthrust fault. This complex hydrogeological system implies the separation of water masses belonging to two di erent creeks: the for- mer, where the main laboratories are located, ows in well drained cretaceous formation, while the latter is within not drained and poorly permeable dolomitic formations. Depending on the path and on percolation features, water masses are therefore characterized by di erent chemical-physical properties. These physical observables are constantly monitored since June, 2008. The rst part of our analysis is focused on the identi cation of periods of sei- smic anomalies, to discriminate between seismic activity and moderate seismicity. We consider the Italian Seismological Instrumental and Parametric Data-Base by INGV and we select events occurred between April 16th, 2005 and September 30th, 2014 with Ml 1.0, in the circular region centered at LNGS-INFN with a radius of 100 km. The catalog is tested for being homogeneous and complete. The leading role in the seismic analysis is played by the energy release Ei, estimated for each i-th seismic event with magnitude Mi and normalized to the minimum energy release Emin, relative to the minimum value of magnitude Mmin considered. The normalized energy release is calculated for monthly sliding time windows, together with the number of events. Periods of intense seismic activity are de ned as those time intervals when both E and N exceed the threshold corresponding to the 30% of the considered time interval. Thus, four periods of intense seismic activity are identi ed. The second part of our work is based on the characterization of uranium time series. 1 The uranium contents are revealed by ICP-MS technique (using a 7500a from Agilent Technologies). We describe the process of optimization of a Fast Fourier Transform (FFT) algorithm. Such a script is developed and enforced to derive information about the main features of the uranium time series. The characteristic frequency intervals are revealed and ltered. As a nal step, the outliers of the residual time series are identi ed. The analysis is applied to the time series of uranium contents of groundwater sampled in sites E1, E3, E3dx and E4, displaced inside the underground facilities. Our main result is the observation of an anomalous behavior of uranium con- tent just before the L'Aquila earthquake, in every sampling site considered. Some cross-correlations between outliers are found. Furthermore, we observe an anoma- lous value before the seismic swarm that a ected the region under consideration between July and December, 2010, only in the sampling site E3dx. No anomalous value in uranium content is observed during the seismic activity time spans. We propose that the uranium groundwater anomalies observed before the sei- smic swarm and the main shock, which occurred on April 6th, 2009 in L'Aquila, provide a key geochemical signal of a progressive increase of deep CO2 uxes at middle-lower crustal levels. Repeated sharp U enrichments in groundwater, that can be directly associated with the geodynamics of the earthquake, represent a much more precise strain-meter than radon. Indeed, radon concentration depends on its parent nuclides uranium and radium during the preparation phase of the earthquake (and on their geochemical patterns with reference to environmental redox and pH characteristics) and it is only successively released by microfrac- turing, during the main shock and aftershocks (see Plastino et al., 2011 and references therein). We believe that uranium anomalies could be used as a possible strain meter in domains where continental lithosphere is subducted.